It is well known that a coaxial transmission line with a movable probe may be used as a component of an overall apparatus for measuring the impedance of a load. The present invention relates to this form of coaxial transmission line.
A coaxial transmission line must, of course, have suitable supporting means for supporting the inner conductor. The supporting means may take the form of spaced supports such as beads, or dielectric pins. At points close to the generator end of the transmission line reflection from the supporting means is not a serious problem but at points further along the line toward the other end, the supportng means becomes of primarily importance because reflections from said supports are directly additive, in vectorial fashion, to the waves that are intended to be measured. Groups of radially disposed pins are often used as the supporting means, in which case all of the pins of a given group are located in a single plane perpendicular to the inner conductor, the groups being spaced apart in order to support the inner conductor along its entire length. To avoid reflections from a dielectric pin, various prior art techniques have been used. For example, holes may be drilled in the inner conductor, longitudinally forward of and/or rearward to the pin, while maintaining a certain distance from the pin, to provide inductive reactance opposing the capacitive reactance of the pin (see U.S. Pat. No. 2,796,589). Alternatively, one or more circumferential grooves spaced longitudinally ahead of and/or behind the pin have been used, the grooves however being spaced from the pin. It has also been proposed to have cavities entirely enclosed within the inner conductor which are longitudinally ahead of, as well as behind, a group of pins, to provide the necessary compensation (see Bondon U.S. Pat. No. 3,151,925 entitled "COAXIAL TRANSMISSION LINE UTILIZING REACTANCE COMPENSATED, PAIRED PIN-TYPE INSULATOR SPACING ASSEMBLY", dated Oct. 6, 1964; and Bondon U.S. Pat. No. 2,589,328 entitled "COAXIAL TRANSMISSION LINE SPACING ASSEMBLY", dated March 18, l952).
All of the above arrangements for compensating for the capacitive effect of the dielectric pin have their drawbacks, including manufacturing problems, and this is especially true as the coaxial transmission lines are made in small sizes so as to be suitable for use at frequencies up to as high as 40 GHz.
It is, therefore, an object of this invention to provide improved compensation for the capacitive effect of a dielectric pin used to support the inner conductor of a transmission line.
It is a further object of the invention to provide a coaxial transmission line suitable for use at frequencies up to 40 GHz.
An additional object is to provide a coaxial transmission line which may be easily assembled.
It is also an object of the invention to provide a coaxial transmission line that may be manufactured at low cost.
It is still another object of the invention to provide a slotted transmission line with a probe, suitable for measuring the impedance of a load, which may efficiently operate at frequencies up to as high as 40 GHz and yet be capable of manufacture at low cost.